CTA Blue Line service was back to normal just after 6 a.m. after hours of delays from a broken track.
Transit Authority crews fixed approximately 1,500 feet of track on the northbound side near Harlem Avenue early Tuesday between Cumberland and Jefferson Park stations. Earlier the trains were running in both directions using the same track.
Officials said a CTA train heading toward O'Hare Airport around 11:30 p.m. Monday experienced a track issue.
The CTA said the break may have been caused by cold weather.